1. Field of the Invention
The present invention relates to a tunable multimode wavelength division multiplex Raman pump and amplifier, and a system, method, and computer program product for controlling the same.
2. Discussion of the Background
With the explosion of the information age has come a demand for larger data transmission capacity for optical communication systems. Conventionally, optical communication systems transmitted data on a single optical fiber using a single wavelength of light (e.g., 1310 nm or 1550 nm). Signals at these wavelengths were desirable since they have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of these single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route which greatly increased the cost of optical fiber networks.
To mitigate this problem, wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system have become desirable. In a WDM system, a plurality of optical signals, each having a different wavelength, can be transmitted simultaneously through a single optical fiber.
Optical fiber communication systems transmit optical signals over considerable distances. However, the signal strength of the optical signals attenuates with distance because of absorption and scattering. Signal strength attenuation ultimately results in signal reception degradation if the signal strength is not kept above background noise (or other sources of noise) by a predetermined amount. Amplifiers are used to keep the signal strength above background noise by a predetermined amount. In general, there are two approaches to amplifying an optical signal: the first, is to use an electronic repeater, which converts the optical signal into an electric signal, amplifies the electrical signal, and then converts the amplified electrical signal back into an optical signal for further transmission along an optical fiber; the second, is to amplify the optical signal itself. Two types of amplifiers that can be used to amplify an optical signal according to the second approach are rare earth doped fiber amplifiers such as erbium doped fiber amplifiers (EDFA), and Raman amplifiers.
EDFAs are currently the most widely used optical amplifiers for WDM systems and are effective and reliable for optically amplifying WDM signals. However, EDFAs have an amplification bandwidth that is limited in range, and produce a wavelength-dependent gain profile. These two characteristics of EDFAs are undesirable for WDM signals, which are spectrally distributed, since a non-uniform amount of gain will be applied to the various WDM channels, depending on the wavelength of the channels. To offset this effect, a gain flattening filter may be used to obtain a uniform or flat gain profile (having a gain deviation of less than 1 dB) across the entire communication band. The gain flattening filter is designed to have a loss profile having a shape that is the inverse of the shape of the gain profile. Gain flattening filters, however, are limited to a particular gain profile, and are not dynamically adjustable to compensate for changes in a magnitude of the gain of the EDFA. Therefore, a flat gain profile cannot be maintained when the gain of the EDFA is changed, or if the attributes of the communications network are changed, such as by adding more WDM signals. In addition, the gain flattening filter decreases the total amount of power launched into an optical fiber.
Raman amplifiers use a phenomenon known as Stimulated Raman Scattering (SRS) of light within an optical fiber to achieve a gain in a particular wavelength band. The inelastic scattering process generates an optical phonon and a co-propagating Stokes wave, light that is downshifted in frequency from the pump light by an amount equal to the phonon frequency (i.e. total energy is conserved). In silica fibers, the peak SRS gain occurs at about 13 THz below the pump light frequency (or conversely, at a wavelength that is longer than a wavelength of the light pumped into the optical fiber by about 100 nm). Since Raman amplification is a scattering process, unassociated with the resonance properties of any particular material, one can generate a Raman gain spectrum for pump light at any wavelength. Therefore, changing a wavelength of the pump light, changes the wavelength at which a peak gain is applied to WDM signals, thereby amplifying some WDM signals more than others. By multiplexing several different pump wavelengths into the same fiber, one can generate a reasonably flat gain spectrum over an arbitrary bandwidth. Because Raman amplifiers require a greater pumping power to obtain the same gain as an EDFA, Raman amplifiers have primarily been used in signal wavelength bands outside of the amplification bandwidth of EDFAs.
Although a Raman amplifier amplifies a signal over a wide wavelength band, the gain of a Raman amplifier is relatively small and, therefore, it is preferable to use a high output laser device as a pumping source. However, increasing the output power of a single mode (or frequency) pumping source beyond a certain threshold leads to undesirable stimulated Brillouin scattering and increased noise at high peak power values. As recognized by the present inventors, to prevent this problem, a multimode laser device is preferably used as a pumping source in a Raman amplifier. A multimode laser has a plurality of oscillating longitudinal modes, each providing output power at less than the threshold at which stimulated Brillouin occurs. A multimode laser can provide a sufficient amount of output power to achieve Raman amplification distributed over the various modes (i.e., wavelengths of output light), as opposed to providing the power all at a single wavelength.
To control the wavelength of the light emitted from the pumping source, and therefore, determine what wavelength of signal will be amplified, it is well known to use fiber gratings. A fiber grating selectively reflects certain wavelengths of light causing a laser beam of a specific wavelength to be output. Fiber gratings are known to be included in the core of an optical fiber, separate from the laser device itself.
Spectroscopy is one application for using a tunable fiber grating is described in U.S. Pat. No. 6,188,705, the entire contents of which being incorporated herein by reference. A tunable fiber grating is described as being coupled to a quasi-monochromatic light source for selecting a single frequency of light output from the light source. Although the light source is capable of producing multiple modes of light, the tunable fiber grating is configured so that only a single frequency output of the light source results. As explained in U.S. Pat. No. 6,188,705, for spectroscopy applications, signal frequency operation is highly desirable, if not required, in order to detect substances with narrow bandwidth absorption lines (""705 patent, col. 1, lines 54-57). A wavelength tuning mechanism may tune the fiber grating by way of temperature change, compression, or through the application of stress of strain (""705 patent, col. 6, lines 14-42).
The present inventors have recognized, however, that the single frequency aspect of the tunable fiber grating described in the ""705 patent would render it unsuitable for use as a Raman amplifier pump device because, as discussed above, multimode pump sources are superior to single frequency pump sources when used in Raman applications.
As described in Bruce, E. xe2x80x9cTunable Lasers,xe2x80x9d IEEE Spectrum, pages 35-39, February 2002, there are a variety of other types of tunable lasers made for use in WDM systems, although the primary application is for generating a WDM signal at a particular frequency (or wavelength). As recognized by the present inventors, since none of these tunable lasers are made for operation as a Raman pump source that intentionally outputs light at more than one frequency, it is unclear from the literature how, or whether, these devices could be adapted for use in multimode applications.
As described in U.S. Pat. No. 6,292,288, in order to achieve a uniform gain profile over a broad range of wavelengths, a Raman amplifier can include multiple pump lasers, each providing multimode light having a predetermined spectral width, centered at a different central wavelength. By properly spacing in wavelength the pump lasers with predetermined optical output levels, it is possible to achieve a composite gain profile that is flat over a broad range of wavelengths, and therefore to provide Raman amplification over a broad range of wavelengths.
U.S. patent application Ser. No. 09/775,632 describes a system through which Raman amplification performance can be controlled. As described in that application, by controlling an output power of each of the lasers of a particular Raman amplifier, the desired gain characteristics can be maintained. Moreover, by monitoring and controlling a portion of a network, the Raman amplification performance of that portion can be controlled through cooperative adjustments made to one or more of the Raman amplifiers (or of individual pump lasers of a particular Raman amplifier) that impact that portion of the network.
FIG. 1 is a block diagram of a conventional Raman amplifier 100. The Raman amplifier 100 includes an amplifier fiber (optical fiber) 103, a WDM coupler 104, a pumping device 107, a control unit 119, and optional polarization independent isolators 102, 105. The Raman amplifier 100 is connected (or merely coupled) to an input fiber 101 and an output fiber 106, which may be optical transmission fibers such as single mode fibers (SMF), dispersion compensation fibers (DCF), dispersion flattening fibers, etc.
The Raman amplifier 100 is connected to a network 122 via a communication link 123. The network 122 is also connected to other amplifiers 124, 125 fiber as well as a remote device controller 121. The remote device controller 121 monitors the operational status of the Raman amplifier 100 as well as the other amplifiers 124, 125. The network 122 may be a proprietary wireless or wired network, or another network that is publicly accessible, such as the Internet or a hybrid network, part proprietary and part publicly accessible. While the Raman amplifier 100 may operate autonomously, it may receive additional information about the overall system performance, such that the control unit 119 can adapt the amplification performance of the Raman amplifier 100 to help offset any adverse affects to the system""s performance, as might be necessitated by a change in conditions, described in the additional information. As an example, this additional information may be that a replacement fiber with different attenuation characteristics is being used to interconnect two cascaded Raman amplifiers in a WDM system. In this case, the Raman amplifier 100 may set a new xe2x80x9ctargetxe2x80x9d amplification performance so as to normalize the channel characteristics for all of the WDM channels, despite the fact that the new fiber may attenuate some of the channels by a lesser amount than others.
The pumping device 107 includes Fabry-Perot type semiconductor lasers 109, 110, 111, 112, wavelength stabilizing fiber gratings 113, 114, 115, 116, polarization couplers 117, 118, and a WDM coupler 108. The central wavelengths of the semiconductor lasers 109 and 110 and wavelengths of the fiber gratings 113 and 114 are the same wavelength xcex1, and the central wavelengths of the semiconductor lasers 111 and 112 and reflection wavelengths of the fiber gratings 115 fiber and 116 are the same wavelength xcex2. The central wavelengths of the semiconductor lasers 109, 110, and 111 and 112 are respectively stabilized to xcex1 and xcex2 via the wavelength stabilizing fiber gratings 113, 114 and 115, 116.
Multimode light generated by the semiconductor lasers 109, 110 and 111, 112 is combined by polarization combiners 117, 118 for each central wavelength xcex1 and xcex2, respectively. The light output from the polarization combiners 117, 118 is combined by the WDM coupler 108. Polarization maintaining fibers 126 are used in the connections between the semiconductor lasers 109, 110, 111, 112 and the polarization combiners 117, 118 to maintain two different polarization planes. This ensures that an input signal to the Raman amplifier 100 will be adequately amplified regardless of its orientation in the signal fiber 101 or amplification fiber 103.
The pumping device 107 in this example includes two pumps that provide light having two different wavelengths xcex1 and xcex2 to the amplifier fiber 103 (i.e., a first pump that provides light having a central wavelength of xcex1, and a second pump that provides light having a central wavelength of xcex2). Further, as noted in U.S. Pat. No. 6,292,288, a wavelength interval between the wavelengths xcex1 and xcex2 is selected to be in a range of 6 nm to 35 nm in order to provide a flat gain profile over a range including both xcex1 and xcex2.
The light output from the pumping device 107 is coupled to the amplifier fiber 103 via the WDM coupler 104. An optical signal (e.g., a WDM signal) is incident on the amplifier fiber 103 via the input fiber 101. The optical signal is then boosted in signal level after the gain medium has being excited by the light pumped into the amplifier fiber 103, the net result being that the optical signal is Raman-amplified. In addition, the Raman-amplified optical signal is passed through the WDM coupler 104 and is transmitted toward the control unit 119, where a part of the amplified optical signal is branched to form a monitor signal (or sampled output signal), while the majority of the signal is output on the output fiber 106.
The control unit 119 includes a processor to assert control over the amplification performance of the Raman amplifier. The control can be based on either the monitored signal or an external source, such as, for example, a control signal received from the remote device controller 121. The control unit 119 generates a control signal on a bus 120, that includes a sufficient number of control lines, so as to allow for control of the drive currents and for the individual semiconductor lasers 109, 110, 111, 112 to achieve a small gain deviation relative to a target gain profile (e.g., a flat amplification profile).
As compared with EDFAs, for example, Raman amplifiers are more complex devices since they contain more laser diode modules, operate over wider bandwidths that are determined by system parameters, and require controllers that are able to establish predetermined amounts of gain across the amplification bandwidth, consistent with network requirements. As recognized by the present inventors, part of the complexity is manifested in a controller that is able to adjust pump output levels when environmental or network requirements change. For example, the central wavelengths of the pump modules will change as a function of temperature. This change in central wavelength will result in change in gain shape, which must be detected by the controller and compensated. However, changing pump power level also effects the Raman amplifier""s gain characteristic, and thus optimum control is not always possible due to temperature induced wavelength shifts in pump light.
Likewise, changes in system requirements may create a situation where the amplification bandwidth of the amplifier must be changed (i.e., widen, or shift to another band). While some changes are possible by switching-in or switching-out pumps to accommodate bandwidth changes, this leads to more expensive amplifiers, because more pumps are needed in the amplifier, albeit not used until needed. Likewise, some of the pumps may degrade overtime or fail. On-board spares may be used to mitigate reliability concerns, however this solution is expensive to implement if all pumps are provided with an on-board spare.
The addition of channels within the amplification bandwidth is another possible scenario. In such a scenario, it may be necessary to increase amplifier power to avoid pump depletion. Since Raman scattering is a non-linear process, the amplifier gain cannot be increased by simply increasing the laser pump power, since doing so will change the power partitioning in the output wavelengths, which will likely result in a non-flat gain spectrum. Furthermore, the non-linear gain characteristics may give rise to undesirable four-wave mixing products, which may result in in-band (signal band) noise spurs that compromise signal to noise (or spurious) ratio requirements. As recognized by the present inventors, the four-wave mixing products are the result of operating at too high of an input pump light level at a particular pump central wavelength. When this occurs, the amplification characteristic of the gain medium at that central wavelength is substantially non-linear, and undesirable spurious responses are created. Given the number pumps that operate at different wavelengths in a Raman amplifier, the risk of four-wave mixing can be intractable if not detected and controlled by reducing the pump levels. However, if reducing the pump levels results in insufficient amplification, the problem can be circumvented by reconfiguring the pump wavelengths in such a way that they provide sufficient gain with sufficient flatness, and their four-wave mixing products are either suppressed or lie out-of-band.
The inventors of the present invention have recognized that conventional optical communication systems are limited as to their flexibility and adaptability. Accordingly, one object of the present invention is to provide a tunable multimode wavelength division multiplex Raman pump and amplifier, and a system, method and computer program product for controlling the same, which address the above-identified and other limitations of conventional systems.
In order to achieve flat gain (or an arbitrary gain shape, such as to compensate for a non-flat fiber attenuation characteristic) for a Raman amplifier, as discussed above, the wavelengths and power levels of each Raman pump are carefully chosen. The present inventors have recognized that, unfortunately, the term xe2x80x9cflat gainxe2x80x9d can mean different things depending on the particular characteristics of a given communications system. Furthermore, the present inventors have recognized that the requirements for a particular system may change over time. For example, new channels may be added or dropped at different wavelengths, bit rates may be increased, or in-line components that are not easily replaced may fail. A one-size-fits-all approach to Raman pumping does not exist. The inventors of the present invention have recognized that it would be advantageous to have a Raman pump module that can be reconfigured by tuning the central wavelength and setting the optical output of each Raman pump individually, allowing the controller to alter the gain profile as needed. Accordingly, another object of the present invention is to provide a tunable multimode Raman pump and a tunable Raman amplifier that uses tunable stabilizer fiber Bragg gratings (FBG) to shift the reflection bandwidth. By providing a tunable Raman pump and a tunable Raman amplifier, more flexible optical communication systems can be built that can be controlled to adjust to either changes in the requirements of the network, or to sub-optimal performance caused by a variety factors.
To achieve these and other objects, the present invention provides a tunable multimode WDM Raman pump, amplifier, control system, method and software that uses a plurality of multimode pumps, whose optical outputs and central wavelengths are controlled by a control unit. Controlling both the wavelength and optical output of the pumps to predetermined levels and/or wavelengths enables a flexible approach toward Raman amplifying a WDM optical signal that propagates through the optical fiber that serves as the Raman gain medium. The control unit ensures that the Raman amplification profile (e.g., a predetermined amplifier gain profile across the amplification bandwidth, and/or the amplification wavelength span) is set and maintained to be consistent with system requirements.
The control unit monitors the amplified WDM signal and, subsequently, determines if the monitored amplified WDM signal is within a predetermined threshold of the target amplification profile. If the Raman-amplified signal is not within the predetermined threshold, the control unit actively controls the pumps (by adjusting at least one of the optical output and central wavelength) to bring the monitored amplified WDM signal within the predetermined threshold of the target amplification profile. The control of the individual pumps may include adjustments made to the output power of the pump and/or the output wavelength of pumping light provided by the pump.
The control unit is also configured to respond to control signals from an external source (e.g., an central controller or other source) that directs the Raman amplifier to create a new target amplification profile. This new target amplification profile may be based on, for example, a change in system operating conditions or system requirements.
The control unit is also configured to monitor for presence of four-wave mixing by observing in-band noise products that are more narrowband than background noise. When detected, the control unit adjusts the central wavelengths of the contributing pumps so as to xe2x80x9csteerxe2x80x9d the narrowband noise out of the signal band.
In one embodiment of the present invention a reconfigurable Raman pump module includes a multiplexed array of wavelength tunable laser diodes. Tuning is achieved by uniformly shifting the reflection bandwidth of the pump""s stabilizer FBG. This approach allows for a single module design that is applicable for many different systems, thereby reducing the manufacturing and inventory problems associated with custom-built equipment. It also reduces the costs associated with future system upgrades.
One embodiment of the reconfigurable Raman pump includes a semiconductor laser diode with an electronic input optically coupled to a waveguide output. A grating structure is inscribed in the waveguide output to provide a small amount of feedback to the diode. The grating is coupled to a tuning mechanism that changes either the grating period, the effective index of the waveguide in the grating region, or both. The reflected feedback from the grating causes the lasing linewidth (or bandwidth of optical output) of the laser diode to roughly correspond to the grating bandwidth. When the grating is tuned, its reflection bandwidth uniformly shifts, resulting in a corresponding uniform shift in the multimode output of the laser diode, thereby achieving tunability of the Raman pump.
The tuning of the grating in the context of the present invention may be through, for example, thermal effects or strain effects (compressive or tensile). Thermal tuning works by inducing changes in the effective index of the waveguide, known as the thermo-optic effect. Changes in the grating period due to the thermal expansion of the waveguide are a second order effect, at least for silica-based materials. Strain tuning works by both changing the effective index via the stress-optic effect and causing small changes in the grating period.
Another feature of the present invention is that each Raman amplifier in an optical communication system need not operate alone, but rather may operate in an internetworked fashion with other amplifiers in the system. Since Raman amplification is a distributed amplification, the present invention exploits this distributed effect by shifting amplification duties between adjacent, cascaded Raman amplifiers so as to compensate for unforeseen changes in component operations or system requirements.
Consistent with the title of this section, the above summary is not intended to be an exhaustive discussion of all the features or embodiments of the present invention. A more complete, although not necessarily exhaustive, description of the features and embodiments of the invention is found in the section entitled xe2x80x9cDESCRIPTION OF THE PREFERRED EMBODIMENTS,xe2x80x9d and more generally throughout the entire document.